Magnetic Resonance Imaging (MRI) can generate cross-sectional images in any plane (including oblique planes) of the human body. Medical MRI most frequently relies on the relaxation properties of excited hydrogen nuclei (protons) in water and fat. When the object to be imaged is placed in a powerful, uniform magnetic field the spins of the atomic nuclei with non-integer spin numbers within the tissue all align either parallel to the magnetic field or anti-parallel. The output result of an MRI scan is an MRI contrast image or a series of MRI contrast images.
Many neurological diseases, such as Alzheimer's disease or multiple sclerosis (MS), lead to brain atrophy, i.e. a loss of brain tissue volume in a faster rate than normal. It is interesting to monitor the brain volume evolution of these patients having such diseases to determine the severity of the disease and the impact of treatment. Generally the brain volume is normalized with the intracranial volume to minimize the effect of head size or incomplete acquisition coverage with the imaging modality. The ratio of the brain parenchymal volume (BPV) and the intracranial volume (ICV) is called the brain parenchymal fraction (BPF) and is considered a measure for brain atrophy (see e.g. Grassiot B, et al. Quantification and clinical relevance of brain atrophy in multiple sclerosis: a review. J Neurol 2009; 256:1397-1412).
Further, neuromuscular diseases may cause muscular dystrophy by cell atrophy in the muscular tissue. Therefore, monitoring the gradual change in muscle tissue volume may be of interest to determine the severity of the disease and the impact of any treatment being performed.
In addition to that, monitoring changes in other types of tissues such as internal organs, e.g. liver, kidneys and so on may also be of interest. In fact, such monitoring may be of use concerning all types of soft tissues, i.e. tissues of the body which are not hard tissue such as bone.
An issue for monitoring patients is that tissue may be under or over hydrated. With a reduction of water content the measurable tissue volume will decrease and with a surplus of water content the measurable tissue volume will increase. The hydration state may vary in time, therefore providing an additional variable that obscures the ‘true’ volume of the tissue. This issue may affect monitoring brain atrophy in neuro-degenerative diseases: a brain may for example be swollen due to inflammatory processes or drinking of the patient. Another example is muscle atrophy in musculoskeletal diseases, where muscle volume may appear to decrease due to dehydration of the patient.
To be able to estimate non-aqueous tissue volume of an object while taking into account the above mentioned drawbacks would therefore be desirable.